This invention relates to advanced control systems for injection molding such as supplied by the Farrel Company, Division of USM Corporation, Ansonia, Conn. These controls may be said to be extensions of conventional control system concepts. In this regard, conventionally, a number of direct process variables such as melt temperature and shot volume are adjusted indirectly by manipulating or adjusting the various direct machine controls such as position limit switches, speed regulators, and timing devices which may be directly mounted upon the machine to effect these variables. Other direct process variables such as shot holding time and in mold time are conventionally controlled by timer without being controlled positively in direct relation to specific events within the injection process. In the advanced control system subsequently described, conventional machine sensors and actuators affecting the various events in the total process have been repositioned, replaced and supplemented, and the machine controls have been sequentially readjusted and made interresponsive to provide an overall control of the injection molding procedure not previously available.
Among the overall objectives of the control system, the advance control system is to control certain key process variables individually, or directly, by individually sensing the process parameters and generating appropriate feed-back signals, and then using these signals to initiate or perform the specific control task for adjustment of the process. This, to a large extent, enables the usual machine process control decisions to be taken out of the hands of manual operators and provides a system which produces an acceptable product under semi-automated and varied operating conditions. The advanced control system contemplated herein may be broken down into several subsystems performing individual or localized process control functions; and among these are melt temperature control, shot volume control and various monitoring of "remote" control process variables.
The present invention relates in general to method for controlling the plasticating or polymeric materials in injection molding processes. More specifically, this invention relates to (1 ) control of the heat energy supplied to or generated within the plasticating apparatus for melting and plasticating the material; (2) control of the quantity of plastic material subsequently injected into a mold associated with the machine; (3) sequential and quantitive control of the various pressures exerted on the plastic material during the injection process; and (4) the flexibility to provide coordinated adjustment of all of the above in response to product evaluation signals when direct input of these values is available.
The specific invention disclosed herein relates to the integrated control of the temperature of the barrel of the plasticator within which the polymeric material is being melted and mixed, in conjunction with the control of the rotational speed and back pressure of the plasticating screw within the apparatus which performs the actual plasticating function. The specific invention disclosed also relates to control of the stroke of the plasticating screw so that it will, first, draw back during mixing to a position providing exactly the shot volume desired, secondly, inject the charge at the desired velocity or pressure, thirdly, sequentially shift through the various pressures necessary to fill, pack and cure the material in the mold, and finally repeat the sequence.
It has been recognized that the plastic materials used in the injection molding processes today often require that the melt temperature be accurately controlled in order to carry out the injection molding process effectively. If the plastic material is either below or above the required temperatures the material may be incompletely plasticated or it may become discolored or otherwise deteriorated before or during injection molding procedure.
A further complication in the control of the temperature of polymeric material being plasticated is that the rate of heat transfer through the material is quite low and efforts to supply make-up heat from external sources such as through direct heating of the barrel or plasticating chamber tend to cause the material to deteriorate in the area of the interface between the material and the barrel itself at a time long before substantial heat has been conducted into the more central portions of the plasticating mass.
No controls existed prior to our invention to adjust and insure proper temperature of the melted, plasticated mass and to insure effective injection by a straightforward, sequential event-related approach. In many of the conventional plasticating and injection molding apparatus, the actual temperature of the mass injected is not measured or coordinatedly controlled during the continuing, repeated steps of the injection molding process. Much of the effort of control has been directed to a trial and error approach, finding acceptable values and then running a machine with these set values, with no provision for feed-back signals to check the set values. Specifically, these methods often only employ nominal temperature correction subsequent to the molding process consistent with visual observations of ineffective molding such as the aforementioned color changes or other material deterioration or the improper filling of a mold or other visible indications of unmixed or poorly mixed injection materials.
While these conventional machines have been somewhat satisfactory for manual operation on relatively low volume bases, the control methods employed therein have been totally unsatisfactory for high speed automatic molding machines. More particularly, the conventional control approaches would be totally unsatisfactory for a coordinated computer controlled machine which would be capable of detecting the individual imperfections in the molding process and product and making the appropriate on line corrections and adjustments in the process to effect a satisfactorily molded product.
Resolution of the shot to shot problems involved in control of the melt temperature, however, do not individually provide the necessary speed of production in control over the production process to bring about the economy, efficiency and quality of production necessary for automated, high speed production. That aspect of the system should be coupled to responsive event control of shot volume and a continuing monitor/control of the injection stroke of the plasticating screw.
Conventional methods of controlling the volume of material injected into a mold by an injection molding machine include various timing means, position limit switches and position control switches influencing the screw backstroke to control the amount of polymeric material collected within the mixing chamber and then injected into the mold. It should be recognized that these conventional controls systems utilize input commands which are not indicative of the particular process parameters, or true events of the process. These conventional controls merely initiate step-by-step functions set by trial and error. By such conventional control systems the various process pressures and flow rates developed within the system are controlled by manual adjustment of relief valves and flow control valves, timing devices and the like.
Such open loop control systems do not lend themselves readily to fully automatic control systems wherein the particular processes performed are controlled in response to deviations from the particular desired process parameters. Some attempts at semi-automation have been made by setting or adjusting these otherwise manually controlled devices by means of a punch card system which inputs certain commands into the process machine in certain desired sequences. These "refined" systems however are based on operator controlled systems inputting changes to the mentioned limit switches or time functions and not the actual process control, thereby effecting process operation by predetermined commands unresponsive to existing conditions.
While these conventional machines have been somewhat satisfactory for operation on relatively low volume basis, the control methods employed therein are totally unsatisfactory for a high-speed, fully automated molding machine. More particularly, the conventional control approaches would be totally unsatisfactory for a computer controlled machine including sensors capable of detecting the imperfections in its molding processes and products, and making appropriate corrections by adjustments in the particular process parameters which make up the molding process.
The present invention, then, relates to method for incrementally adjusting the temperature generated within one charge of the injection molding process, by coordinatedly controlling the speed of rotation of the screw, the back pressure of the screw and the barrel temperature of the plasticating chamber. In coordination with this, the invention includes method and apparatus for continually controlling the volume of the shot injected into the mold in the machine, and in addition the coordinated control of the sequential injection stroke of the screw according to the pressure and velocity patterns of the melt and screw during the injection process.